1. Field of the Invention
This invention relates generally to the field of electromagnetic devices, and more particularly to cooling of electromagnetic devices such as power transformers.
2. Brief Description of the Prior Art
A basic electrical transformer consists of two or more conductive coils wound around a common magnetic core. When a time-varying voltage is applied across one (“primary”) coil, a corresponding time-varying voltage is produced in the other (“secondary”) coil through the property of magnetic induction. By adjusting the number of turns in the windings of the secondary coil relative to that of the primary coil (the “turns ratio”), the time-varying voltage induced across the secondary may raised or lowered relative to that of the primary. Transformers are commonly used in power electronics, for example, to convert to the electrical energy provided by a power source to voltage levels required by a particular load.
Because all real coil and core materials have imperfect electrical and magnetic properties, energy is lost and dissipated in the transformer elements in the form of heat. A means for removing this heat is generally required, and particularly when a transformer is located within an enclosure in proximity to other electrical components that may be impaired in their performance or damaged by high temperatures. This problem is particularly acute in power delivery applications, where generation of high currents results in large amounts of dissipated power, and where an increase in the size of transformer elements results in more heat-generating volume in the element relative to the surface area from which heat may be extracted. The problem is further compounded in high frequency applications due to the fact that magnetic core materials suitable for high frequency operation, e.g. ferrites, tend to have relatively poor thermal conductivity, making heat extraction from the material all the more difficult.
A number of approaches have been suggested for improving heat extraction from transformer elements. For example, Rauls, et al., in “Design Considerations for High Frequency Coaxial Winding Power Transformers,” IEEE Transactions on Industry Applications, Vol. 29, No. 2, March/April 1993, conclude that a low-loss coaxial transformer design having an aspect ratio that is long and thin results in improved heat transfer due to the increased surface area of the transformer cores. They note, however, that heat extraction from the windings of a coaxial transformer is impeded by the surrounding core material, which is generally of poor thermal conductivity, and propose that heat transfer from coaxial transformers may be enhanced by forcing a coolant through the primary electrical conductor.
U.S. Pat. No. 6,087,916 describes coaxial transformer structures having a heat transfer member in contact with both the outer electrical conductor of the transformer and a heat sink, as well as heat conducting straps in contact with the transformer core surfaces. In this configuration, the heat transfer member includes an electrically insulating component if the heat sink is to remain electrically isolated from the transformer. Bendre, et al., also describe a mechanism for cooling a coaxial power transformer in “Design Considerations for a Soft-Switched Modular 2.4-MVA Medium Voltage Drive,” IEEE Transactions on Industry Applications, Vol. 38, No. 5, September/October 2002. There, a coaxial transformer design is illustrated having flat-sided cores that may be placed directly on a baseplate to aid in cooling. To increase further the heat transfer surface area, Bendre et al. suggest that two transformers may be used in a parallel-primary series-secondary configuration.
Another approach to the problem of transformer cooling takes advantage of “planar” transformer designs, wherein the transformer exhibits a reduced height and a correspondingly large footprint area. The windings of a planar transformer may be constructed of flat conductive traces on printed circuit boards, for example, with the resulting transformer profile being very thin and flattened. Thus, the available cooling surface area of a planar transformer may be significantly higher than that of a conventional wire-wound transformer of equivalent volume. In addition, the reduced thickness of the magnetic cores may simplify heat extraction from the core material. At very high power levels, however, the footprint area needed to accommodate a given flux density may become prohibitively large. U.S. Pat. No. 6,222,733 describes a means of improving the cooling of planar transformers using a planar cooling body. U.S. Pat. No. 6,144,276 describes a means of improving the cooling of planar transformers using cooling features integrally formed onto the windings themselves.
In general, a property of both coaxial and planar power transformer designs is the absence of significant leakage inductance; that is, that substantially all of the magnetic flux produced by the primary winding couples to the secondary winding. In some applications, however, the presence of transformer leakage inductance may be desirable. For example, transformer leakage inductance may function as a reactive element in associated circuitry, avoiding the need to add a physical inductor element to perform the equivalent function. U.S. Pat. No. 6,084,499 depicts a high leakage planar magnetic structure having decoupled windings on opposite poles of a common core. The structure has the relatively thin profile and large, flat surface areas typical of a planar core transformer design. The windings, however, are not enclosed entirely within core material, but rather communicate substantially with open air space. No specific cooling means of cooling the structure is described.
U.S. Pat. No. 4,845,606 describes a low leakage transformer design utilizing multiple core elements arranged in a matrix configuration and interwired to function collectively as a transformer. The matrix configuration is described as flat and essentially open in construction, and that cooling of the structure is therefore readily accomplished. The matrix transformer is said to be particularly suited to applications requiring high equivalent turns ratios and high dielectric isolations.
Given the continually increasing demands on power conversion equipment to operate more efficiently and at higher power levels, a configuration permitting improved heat extraction from electromagnetic elements, such as power transformers, would be desirable. It would be further desirable if the improved cooling could be accomplished while retaining a compact and efficient design. It would also be desirable if the improved design accommodated a means of providing for selected values of leakage inductance while minimizing power losses. It would also be desirable if the configuration accommodated a means of actively adjusting leakage inductance so as to provide optimal power sharing among devices operating in parallel or series.